The control of the thrust vector by means of tiltable flaps is known, for example, from U.S. Pat. No. 4,052,007 (Willard). The control flaps are journalled to the jet engine housing or to the aircraft frame or cell. First flaps are operated to tilt these flaps into the exhaust jet, thereby changing the cross-sectional flow area of the exhaust or thrust jet. The nozzle exit configuration in the above mentioned U. S. Patent is rectangular. Thus, two second flaps arranged opposite each other are sufficient for the thrust control. The first flaps are displaceable in guide means for varying the nozzle exit cross-sectional flow area. The two second flaps tiltable about axes rigidly secured to the propulsion plant, are located downstream of the two first mentioned flaps for the jet deflection and/or for the thrust reversal. These additional flaps are tilted substantially in the same direction for the jet deflection. For a thrust reversal these flaps are tilted symmetrically toward each other, that is, in opposite directions. The inner contour of these flaps facing the propulsion jet is substantially straight or flat along most of the flap length, except for a leading edge zone which is slightly curved outwardly away from the flat straight surface of the flap. As shown in FIG. 4 of U.S. Pat. No. 4,052,007 substantial flap angles are necessary when the nozzle exit opening is reduced to its minimum by the upstream, first flaps. The large flap angles are necessary to achieve an effective jet deflection. An effective jet deflection cannot be achieved by smaller flap angles because of the fixed location of the journal axes or tilting axes of these deflection flaps. Tests have shown that such large flap deflection angles are undesirable because if there is an impinging angle of more than 30.degree. between the edge of the jet jet stream and the inner contour of the flap, a partial reverse flow of the jet stream results. An unintended reversal flow can basically be compared with the operation situation of intended thrust reversal, please see also FIG. 6 of Willard's disclosure. Such a thrust reversal, even if it is only partial, is undesirable in connection with the thrust vector control because the jet portion that is deflected by more than 90.degree. blows off toward the leading edge of the flap and causes a thrust loss as well as a cross-force loss. As a result, the propulsion plant or the cell may be exposed to thermal and mechanical loads that are undesirable. In order to avoid dead zones, in other words, to avoid undesirable reaction delays, it is conventional to keep the jet deflection flaps in such a position that they constantly contact the jet edge. As a result, the flaps are subject to substantial thermal loading, although the ejector effect makes possible a limited cooling of the flaps, see FIG. 5 of U.S. Pat. No. 4,052,007.
In spite of such cooling, due to the ejector effect, the thermal loading is so large that only expensive high heat resistant materials can be used for making the flaps.
It is possible to control the flap position relative to all relevant flight maneuvers and relative to all propulsion plant situations. For this purpose it is, however, necessary to ascertain the location of the jet edge and to store corresponding information in the on-board computer for controlling the flap position, or rather adjusting the flap position. The sensing of the jet edge, however, is extremely involved and hence expensive if it is intended to ascertain the jet edge for all necessary flight conditions.